The present invention relates to a flow control method for an Available Bit Rate service in an Asynchronous Transfer Mode network. In particular, the invention is directed to a flow control method using a dynamic Major Reduction Factor (MRF), which is a non-increasing function of queue-length.
Asynchronous Transfer Mode (ATM) networks provide an Available Bit Rate (ABR) service in which the data rate is adjusted based on network congestion information. In the network, each switch employs an algorithm to detect and indicate congestion. One well-known control algorithm is referred to as the Dynamic Max Rate Control Algorithm (DMRCA), described by F. M. Chiussi et al., xe2x80x9cDynamic Max Rate Control Algorithm For Available Bit Rate Service in ATM Networksxe2x80x9d, IEEE Globecom ""96, November 1996, and U.S. Pat. No. 5,701,292, which are herein incorporated by reference.
DMRCA flow control is implemented with an Explicit Rate Manager (ERM) to support ABR service and provide proper flow control in the ATM network. This is achieved by monitoring the queue-length at a congestion point of the switch and the maximum cell rate (MAX) of all connections arriving at the switch. Preferably, MAX is adjusted to smooth excessive oscillations in the maximum rate. The adjusted maximum rate Amax is defined as
Amax=(1xe2x88x92alpha)xc2x7Amax+alphaxc2x7MAX
where
alpha is an averaging factor, for example, {fraction (1/16)}; and
MAX is the maximum cell rate of all connections arriving at the switch.
The DMRCA algorithm compares the queue-length q at a congestion point of the switch to a lower queue-length threshold QT and a higher queue-length threshold DQT. If the queue-length q at a congestion point is greater than or equal to DQT, the switch is considered to be xe2x80x9cheavily congestedxe2x80x9d and the Explicit Rate (ER) field and Congestion Indication (CI) bit of the resource management (RM) cells which deliver the congestion information are marked accordingly. Specifically, the CI bit is set equal to xe2x80x9c1xe2x80x9d and the ER field of the RM cell is set equal to Amaxxc2x7MRF, where MRF is the Major Reduction Factor. If the queue-length q at a congestion point is greater than or equal to QT but less than DQT, the switch is considered to be xe2x80x9cmoderately congestedxe2x80x9d. In this moderately congested case, a marking threshold is set equal to Amaxxc2x7Fn(q), where Fn( ) is a discrete, non-increasing correction function of the queue-length in the range 0xe2x89xa6Fn( )xe2x89xa61, and the calculated marking threshold is compared to the current cell rate (CCR). If the current cell rate is greater than or equal to the marking threshold then the CI bit is set to xe2x80x9c1xe2x80x9d.
The conventional DMRCA flow control algorithm often generates poor transient queue-length performance as a result of using a fixed MRF value to calculate and mark ER. Poor transient queue-length performance is caused by the fact that DMRCA does not explicitly measure bandwidth availability at the congestion point. Instead, DMRCA estimates the transmission rate of each ABR connection at the congestion point based on Amax, which, in turn, is computed based on CCR information carried by the forward resource management (FRM) cell. Since CCR is set by the ABR source when FRM cells depart its source, the CCR information will not change even when the bandwidth available (and thus the actual transmission rate of ABR connections) at the congestion point decreases drastically. In other words, when there is a sudden drop in available bandwidth at the congestion point, e.g., due to increase in high priority traffic, DMRCA will have the wrong information about actual bandwidth availability. Worse still, DMRCA will use the misleading Amax as the basis for major source rate reduction, i.e., ER is set to Amaxxc2x7MRF. Since Amax is significantly higher than the actual fairshare of bandwidth an ABR connection should get after the onset of congestion, Amaxxc2x7MRF is likely to remain higher than the fairshare because the value of MRF is statically set near to 1, e.g. xe2x85x9e. As a result, the queue grows as it is filled with forward RM-cells carrying stale CCR information and the Amax values remain relatively large. The larger the queue becomes, the longer the Amax remains at high values. This positive feedback cycle continues indefinitely. In short, setting ER equal to Amaxxc2x7MRF fails to reduce congestion. It is therefore desirable to develop a scheme to overcome these disadvantages.
The present invention is directed to a method for adjusting the flow rate of available bit rate connections in an asynchronous transfer mode network based on congestion information detected at a switch and conveyed using resource management cells having an explicit rate field. Upon an occurrence of an event, a determination is made of an estimated fairshare of bandwidth allotted to a particular connection at the switch. When a queue-length of the switch is greater than or equal to a queue-length threshold, the estimated fairshare of bandwidth allotted to the particular connection is then adjusted based on a dynamic major reduction factor, which is a non-increasing function of queue-length.
In a preferred embodiment, the maximum flow rate among all connections arriving at the switch is detected upon the arrival of an forward resource management cell at the switch and the detected maximum flow rate is adjusted based on a running exponential weighted average. A determination is made whether a queue-length of the switch is greater than or equal to a queue-length threshold. If the queue-length of the switch is greater than or equal to the queue-length threshold, then the explicit rate field of the resource management cell is marked based on a dynamic major reduction factor, which is a non-increasing function of queue-length.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.